Calcium fluxes in human trophoblast (BeWo) cells: calcium channels, calcium-ATPase, and sodium-calcium exchanger expression

Cadmium inhibits forskolin-induced differentiation of human placental BeWo cells

Cadmium (Cd) is an environmental pollutant. Blood Cd levels in pregnant women have been associated with premature births, infant birth size, placenta previa, and placenta accreta. There have been concerns on the reproductive developmental toxicity of Cd. The choriocarcinoma cell line BeWo, a cellular in vitro model for studying syncytial fusion, has been widely used to study the reproductive and developmental toxic effects of pollutants. Here, we examine the inhibitory effect of Cd against forskolin (FSK)-induced BeWo differentiation. Results showed that Cd exposure inhibited the FSK-induced expression of syncytiotrophoblast-related genes LGALS13, ERVFRD1, SDC1, and CGB3. Inhibition of LGALS13 expression was due to the inhibition of the PKA pathway, whereas the inhibition of the other three genes could be due to the inhibition of the other pathways. These findings could help clarify the reproductive and developmental toxicity of Cd.

Involvement of metal transporters in the intestinal uptake of cadmium

We investigated the mechanism underlying intestinal cadmium (Cd) uptake based on the mediators (metal transporters) of essential elements, such as Fe, Zn, Cu, and Ca, under normal conditions in female rats. These elements interact with Cd uptake from the intestinal tract. Cd concentration at each site of the small intestine (duodenum, jejunum, and ileum) increased as Cd exposure increased. However, Cd concentration was the highest in the duodenum. The gene expression of ZIP14, DMT1, and ATP7A increased with increase in Cd concentration. Further, Cu concentration decreased as Cd concentration increased. In contrast, Fe concentration displayed a decreasing tendency with the increase in Cd concentration. The gene expression levels of ZIP14, DMT1, and ATP7A were positively correlated with Cd concentration. Immunohistochemical staining revealed the positive sites of ZIP14 and DMT1 scattered in the area adjacent to the goblet cells, resorbable epithelial cells, and lamina propria in the duodenum tissue, according to the increase in Cd concentration. Cd is induced to synthesize and bind to metallothionein (MT-I and -II) and accumulate in the intestinal tissues, mainly in the duodenum. Such findings suggest that Cd, a contaminant element, is taken up from the intestinal tract by multiple metal transporters such as Cu, Fe, and Zn, thereby involving in the intestinal Cd absorption.

Concentration-dependent roles of DMT1 and ZIP14 in cadmium absorption in Caco-2 cells

Intestinal absorption of cadmium (Cd) is considered to be mediated mainly by the ferrous iron transporter DMT1, or the calcium transporter CaT1. The roles of zinc transporters such as ZIP8 and ZIP14 remain unclear, and the roles of these four transporters in the intestinal uptake of Cd under physiological conditions have not been compared. Here, we used a trans-well cell culture system to investigate the effects of the down-regulation of these four transporters on the uptake of Cd from the apical side of enterocytes. We used a Caco-2-kh cell line that can form tight junctions within a few days. The transfection of DMT1 siRNA significantly decreased the Cd uptake from the apical side at 5 μM, but not at 0.1 or 1 μM. The transfection of ZIP14 siRNA markedly decreased the Cd uptake at 0.1 and 1 μM, but not at 5 μM. The transfection of siRNA of CaT1 or ZIP8 did not alter the Cd uptake at any concentrations of Cd examined. These results suggest that DMT1 and ZIP14 play different roles in intestinal Cd absorption depending on the concentration of Cd.

Expression and regulation of sodium/calcium exchangers, NCX and NCKX, in reproductive tissues: do they play a critical role in calcium transport for reproduction and development?

Plasma membrane sodium/calcium (Na(+)/Ca(2+)) exchangers are an important component of intracellular calcium [Ca(2+)](i) homeostasis and electrical conduction. Na(+)/Ca(2+) exchangers, NCX and NCKX, play a critical role in the transport of one [Ca(2+)](i) and potassium ion across the cell membrane in exchange for four extracellular sodium ions [Na(+)](e). Mammalian plasma membrane Na(+)/Ca(2+) exchange proteins are divided into two families: one in which Ca(2+) flux is dependent only on sodium (NCX1-3) and another in which Ca(2+) flux is also dependent on potassium (NCKX1-4). Both molecules are capable of forward- and reverse-mode exchange. In cells and tissues, Na(+)/Ca(2+) (and K(+)) gradients localize to the cell membrane; thus, the exchangers transport ions across a membrane potential. Uterine NCKX3 has been shown to be involved in the regulation of endometrial receptivity by [Ca(2+)](i). In the uterus and placenta, NCKX3 expression is regulated by the sex steroid hormone estrogen (E2) and hypoxia stress, respectively. In this chapter, we described the expression and regulation of these proteins for reproductive functions in various tissues including uterus, placenta, and kidney of humans and rodents. Evidence to date suggests that NCKX3 and NCX1 may be regulated in a tissue-specific manner. In addition, we focused on the molecular mechanism involved in the regulation of NCKX3 and NCX1 in mammals, based upon our recent results and those of others.

An outwardly rectifying chloride channel in BeWo choriocarcinoma cell line

In this study, an outwardly rectifying chloride channel was characterized in the trophoblastic cell line BeWo, a human hormone-synthesizing cell which displays many biochemical and morphological properties similar to those reported for the human cytotrophoblast. Ion channel activity was recorded in the cell attached and inside-out configurations with standard patch-clamp technology. In most of the BeWo cells studied, the channel under symmetrical N-methyl-d-glucamine (NMDG-Cl) concentration (Na(+) free solution) in both sides of the membrane exhibited spontaneous activity, an outwardly rectifying current/voltage relationship and single-channel conductances of 15 pS and 48 pS for inwards and outwards currents, respectively. The channel has a low permeability for gluconate with a relative permeability P(gluconate)/P(Cl) of 0.23, and a higher permeability to I(-). The open probability (Po) of the channel exhibited dependence with the applied membrane potential with greater activity at positive pulses. The channel activity was inhibited by the sulphonylurea hypoglycemic agent glibenclamide (50 μM) or by diphenylamine-2-carboxylate (DPC, 500 μM) added to the cytoplasmic side of the patch whereas conductances remained unchanged. The blockade with glibenclamide and DPC was independent of the applied membrane potential. All these results are characteristic of the outwardly rectifying Cl channel (ORCC) found in other types of cells. Neither Po, conductances nor reversal potential (Er) values were affected by the absence of intracellular Ca(2+), suggesting that the channel is not sensitive to Ca(2+).

Epithelial sodium channel in a human trophoblast cell line (BeWo)

The present study was performed to assay sodium currents in BeWo cells. These cells comprise a human trophoblast cell line which displays many of the biochemical and morphological properties similar to those reported for the in uterus proliferative cytotrophoblast. For whole-cell patch-clamp experiments, BeWo cells treated for 12 h with 100 nM aldosterone were exposed to 8Br-cAMP, a membrane-permeable cAMP analogue, to induce channel activity. Cells showed an amiloride-sensitive ion current (IC50 of 5.77 microM). Ion substitution experiments showed that the amiloride-sensitive current carried cations with a permeability rank order of Li+ > Na+ > K+ > NMDG (PLi/PNa = 1.3, PK/PNa = 0.6, PNMDG/PNa = 0.2). In cells pretreated with aldosterone, we observed that nearly half of successful patches had sodium channels with a linear conductance of 6.4 +/- 1.8 pS, a low voltage-independent Po and a PK/PNa of 0.19. Using RT-PCR, we determined that control cells express the alpha-, but not beta- and gamma-, epithelial sodium channel (ENaC) mRNA. When cells were treated with aldosterone (100 nM, 12 h), all alpha-, beta- and gamma-ENaC mRNAs were detected. The presence of ENaC subunit proteins in these cells was confirmed by Western blot analysis and immunolocalization with specific ENaC primary antibodies. In summary, our results suggest that BeWo cells express ENaC subunits and that aldosterone was able to modulate a selective response by generating amiloride-sensitive sodium currents similar to those observed in other human tissues.

Polycystin-2 cation channel function in the human syncytiotrophoblast is regulated by microtubular structures

Polycystin-2 (PC2), encoded by PKD2, which is one of the genes whose mutations cause polycystic kidney disease, is abundantly produced in the apical domain of the syncytiotrophoblast (hST) of term human placenta. PC2, a TRP-type (TRPP2) non-selective cation channel, is present in primary cilia of renal epithelial cells, a microtubule-based ancillary structure with sensory function. The hST has abundant cytoskeletal structures, and actin filament dynamics regulate PC2 channel function in this epithelium. However, it is expected that the apical hST excludes microtubular structures. Here, we demonstrated by Western blot and immunocytochemical analyses that hST apical vesicles indeed contain microtubule structural components, including tubulin isoforms, acetylated alpha-tubulin, and the kinesin motor proteins KIF3A and KIF3B. PC2 and tubulin were substantially colocalized in hST vesicles. Treatment of hST vesicles with either the microtubular disrupter colchicine (15 microM) or the microtubular stabilizer paclitaxel (taxol, 15 microM) resulted in distinct patterns of microtubular re-organization and PC2 redistribution. We also observed that changes in microtubular dynamics regulate PC2 channel function. Addition of colchicine rapidly inhibited PC2 channel activity in lipid-bilayer reconstituted hST membranes. Addition of either tubulin and GTP, or taxol, however, stimulated PC2 channel activity in control hST membranes. Interestingly, we found that the kinesin motor protein KIF3A was capable of increasing PC2 channel activity in hST. We believe that the data are the first to provide a direct demonstration of a microtubular interaction with PC2 in the hST. This interaction thus plays an important regulatory role in the control of ion transport in the human placenta.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

The placental cholinergic system: localization to the cytotrophoblast and modulation of nitric oxide

Background

The human placenta, a non-neuronal tissue, contains an active cholinergic system comprised of acetylcholine (ACh), choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and high affinity muscarinic receptors. The cell(s) of origin of placental ACh and its role in trophoblast function has not been defined. These studies were performed to define the cellular location of ACh synthesis (ChAT) in the human placenta and to begin studying its functional role.

Results

Using immunohistochemical techniques, ChAT was observed primarily within the cytotrophoblasts of preterm placentae as well as some mesenchymal elements. Similar intense immunostaining of the cytotrophoblast was observed for endothelium-derived nitric oxide synthase (eNOS) suggesting that ACh may interact with nitric oxide (NO)-dependent signaling pathways. The ability of carbamylcholine (CCh), an ACh analogue, to stimulate a rise in intracellular Ca⁺⁺ and NO production in trophoblasts was therefore tested using the BeWo^b30 choriocarcinoma cell as a model system. First, CCh significantly increased intracellular calcium as assessed by fluorescence microscopy. We then examined the ability of CCh to stimulate NO production by measuring total nitrite/nitrate production in conditioned media using chemiluminescence-based analysis. CCh, alone, had no effect on NO production. However, CCh increased measurable NO approximately 100% in the presence of 10 nM estradiol. This stimulatory effect was inhibited by 1 (micro)M scopolamine suggesting mediation via muscarinic receptors. Estradiol, alone, had no effect on total NO or eNOS protein or mRNA.

Conclusion

These data demonstrate that placental ChAT localizes to the cytotrophoblast and some mesenchymal cells in human placenta. It further suggests that ACh acts via muscarinic receptors on the trophoblast cell membrane to modulate NO in an estrogen-dependent manner.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Plasma membrane calcium pumps in smooth muscle: from fictional molecules to novel inhibitors

Plasma membrane Ca2+pumps (PMCA pumps) are Ca2+-Mg2+ATPases that expel Ca2+from the cytosol to extracellular space and are pivotal to cell survival and function. PMCA pumps are encoded by the genes PMCA1, -2, -3, and -4. Alternative splicing results in a large number of isoforms that differ in their kinetics and activation by calmodulin and protein kinases A and C. Expression by 4 genes and a multifactorial regulation provide redundancy to allow for animal survival despite genetic defects. Heterozygous mice with ablation of any of the PMCA genes survive and only the homozygous mice with PMCA1 ablation are embryolethal. Some PMCA isoforms may also be involved in other cell functions. Biochemical and biophysical studies of PMCA pumps have been limited by their low levels of expression. Delineation of the exact physiological roles of PMCA pumps has been difficult since most cells also express sarco/endoplasmic reticulum Ca2+pumps and a Na+-Ca2+-exchanger, both of which can lower cytosolic Ca2+. A major limitation in the field has been the lack of specific inhibitors of PMCA pumps. More recently, a class of inhibitors named caloxins have emerged, and these may aid in delineating the roles of PMCA pumps.Key words: ATPases, hypertension, caloxin, protein kinase A, protein kinase C, calmodulin.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

Calcium channels, transporters and exchangers in placenta: a review

Calcium (Ca2+) entry in cells is crucial for development and physiology of virtually all cell types. It acts as an intracellular (second) messenger to regulate a diverse array of cellular functions, from cell division and differentiation to cell death. Among candidates for Ca2+ entry in cells are-voltage-dependant Ca2+ channels (VDCCs), transient receptor potential (TRP)-related Ca2+ channels and store-operated Ca2+ (SOC) channels. Plasma membrane Ca2+-ATPases (PMCA) and Na+/Ca2+ exchanger (NCX) are mainly responsible for Ca2+ extrusion. These different Ca2+channels/transporters and exchangers exhibit specific distribution and physiological properties. During pregnancy, the syncytiotrophoblast layer of the human placenta transfers as much as 30 g of Ca2+ from the mother to the fetus, especially in late gestation where Ca2+ transport through different channels must increase in response to the demands of accelerating bone mineralization of the fetus. The identification and characterization of the different Ca2+ channels/transporters and exchangers on the brush-border membrane (BBM) facing the maternal circulation, and the basal plasma membrane (BPM) facing the fetal circulation; placental membrane of the syncytiotrophoblasts have been the focus of numerous studies. This review discusses current views in this field regarding localization and functions during transcellular Ca2+ entry and extrusion from cells particularly in the placenta.

Functional expression of Ca2+ signaling pathways in mouse embryonic stem cells

Mouse embryonic stem (mES) cells have the potential to differentiate into all types of cells, but the physiological properties of undifferentiated mES cells, including Ca2+ signaling systems, are not fully understood. In this study, we investigated Ca2+ signaling pathways in mES cells by using confocal Ca2+ imaging systems, patch clamp techniques and RT-PCR. The stimulations with ATP and histamine (His) induced a transient increase of intracellular Ca2+ concentration ([Ca2+]i), which were prevented by the pretreatment of 2-amino-ethoxydiphenyl borate (2-APB), a blocker for inositol-1,4,5-triphosphate receptors (InsP3Rs). The application of caffeine (Caff) or ryanodine (Ry) did not change [Ca2+]i. When stores were depleted with Ca2+ -ATPase blocker, thapsigargin (TG), or histamine, the capacitative Ca2+ entry (CCE) was observed. In whole cell patch clamp mode, store-operated Ca2+ currents could be recorded in cells treated with histamine and thapsigargin. On the other hand, voltage-operated Ca2+ channels (VOCCs) could not be elicited. The application of blockers for plasma membrane Ca2+ pump (PMCAs) (carboxeosin or caloxin2A1) induced a large increase of [Ca2+]i. When the Na+/Ca2+ exchangers (NCXs) were blocked by Na+ free solution or KBR7943, [Ca2+]i was also elevated. Using RT-PCR, mRNAs for InsP3Rs type-1, -2, and -3, PMCA-1 and -4, NCX-1, -2, and -3 could be detected. From these results, we conclude that Ca2+ release from ER is mediated by InsP3Rs in mES cells before differentiation and Ca2+ entry through plasma membrane is mainly mediated by the store-operated Ca2+ channels (SOCs). For the Ca2+ extrusion systems, both NCXs and PMCAs play important roles for maintaining the low level of [Ca2+]i.